1. Field of the Invention
The present invention generally relates to material processing and, more particularly, is concerned with an apparatus for controlled processing of materials and a method of controlling hydrocarbon release rate by maintaining target oxygen concentration in discharge gases so as to thereby convert successive batches of materials of widely varying energy content into substantially harmless gases and carbon-free residue ash, such as is necessary for the disposal of medical and other diverse waste material, particularly on-site where the waste material is produced.
2. Prior Art
The problem of disposal of waste matter involves a material processing challenge that is becoming increasingly acute. The primary material processing methods of waste disposal have been burning in incinerators and burial in landfills. These two material processing methods have severe disadvantages. Burning of waste liberates particulate matter and fumes which contribute to pollution of the air. Burial of wastes contributes to the contamination of ground water. A third material processing method is recycling of waste. Although increasing amounts of waste are being recycled, which alleviates the problems of the two primary material processing methods, presently available recycling methods do not provide a complete solution to the waste disposal problem.
The problem of disposal of biomedical waste materials is even more acute. The term "biomedical waste materials" is used herein in a generic sense to encompass all waste generated by medical hospitals, laboratories and clinics which may contain hazardous, toxic or infectious matter whose disposal is governed by more stringent regulations than those covering other waste. It was reported in The Wall Street Journal in 1989 that about 13,000 tons a day of biomedical waste, as much as 20% of it infectious, is generated by around 6,800 U.S. hospitals.
Hospitals and other generators of biomedical waste materials have employed three main material processing methods of waste handling and disposal: (1) on-site incineration with only the residue transferred to landfills; (2) on-site steam autoclaving and followed by later transfer of the waste to landfills; and (3) transfer of the waste by licensed hazardous waste haulers to off-site incinerators and landfills. Of these three main material processing methods, theoretically at least, on-site disposal is the preferred one.
However, many hospital incinerators, being predominantly located in urban areas, emit pollutants at a relatively high rate which adversely affect large populations of people. In the emissions of hospital incinerators, the Environmental Protection Agency (EPA) has identified harmful substances, including metals such as arsenic, cadmium and lead; dioxins and furans; organic compounds like ethylene, acid gases and carbon monoxide; and soot, viruses, and pathogens. Emissions of these incinerators may pose a public health threat as large as that from landfills.
Conventional incinerators most commonly are designed to operate above a certain temperature, such as 1200.degree.-1400.degree. F., to comply with requirements of the permit laws of many states. The reason for this requirement is that conventional thinking has been that operation of incinerators at such elevated temperatures will substantially eliminate the release of most harmful substances. This may have been true where the materials being consumed by the incinerator were assumed to be fairly uniform in terms of energy content and thus burned more or less evenly. However, this is the exception and not the normal situation today, particularly in the case of biomedical waste materials which can range from wet paper towels and steel surgery tools to plastic syringes and containers of saline solution. The thermal processing of these materials by temperature control alone will ordinarily result in the inability to control the hydrocarbon release rate and the repeated emission of un-burned hydrocarbons, typically visible as periodic puffs of black smoke, which is unacceptable under most current environmental regulations.
Nonetheless, on-site disposal of biomedical waste materials still remains the most promising solution. One recent on-site waste disposal unit which addresses this problem is disclosed in U.S. Pat. No. 4,934,283 to Kydd. This unit employs a lower pyrolyzing chamber and an upper oxidizing chamber separated by a movable plate. The waste material is deposited in the lower chamber where it is pyrolyzed in the absence of air and gives off a combustible vapor that, in turn, is oxidized in the upper chamber. While this unit represents a step in the right direction, it does not appear to approach an optimum solution to the problem of biomedical waste material disposal.
One problem with the approach of the aforementioned patent is that it proposes the use of an on-site waste disposal unit which is dedicated to the disposal of biomedical waste material. This approach requires that more than one incineration system be installed and maintained at hospitals, namely, one for biomedical waste and another for all other hospital waste. Resistance has been encountered to the adoption of this approach by hospitals due to added cost of installation, operation and maintenance. An urgent need has developed for an all-purpose material processing apparatus which can handle disposal of all types of hospital waste materials, both biomedical waste and general waste, such as metal needles and glass and plastic bottles.
Reference is also made to the following issued U.S. Patents dealing with subject matter related to the present invention, the disclosures of which are hereby incorporated in their entireties:
1. "Apparatus And Method For Controlled Processing Of Materials" by Roger D. Eshleman and Paul S. Stevers, assigned U.S. Ser. No. 07/987,928 and filed Dec. 9, 1992 and issued U.S. Pat. No. 5,353,719.
2. "Multiple Unit Material Processing Apparatus" by Roger D. Eshleman, assigned U.S. Ser. No. 07/987,929 and filed Dec. 9, 1992, and issued U.S. Pat. No. 5,289,787.
3. "Heat Generator Assembly In A Material Processing Apparatus" by Roger D. Eshleman, assigned U.S. Ser. No. 07/987,936 and filed Dec. 9, 1992, and issued U.S. Pat. No. 5,338,918.
4. "Casing And Heater Configuration In A Material Processing Apparatus" by by Roger D. Eshleman, assigned U.S. Ser. No. 07/987,946 and filed Dec. 9, 1992, and issued U.S. Pat. No. 5,420,394.
5. "Apparatus And Method For Transferring Batched Materials" by Roger D. Eshleman, assigned U.S. Ser. No. 08/026,719 and filed Mar. 5, 1993, issued U.S. Pat. No. 5,338,144.
6. "Sloped-Bottom Pyrolysis Chamber And Solid Residue Collection System In A Material Processing Apparatus" by Roger D. Eshleman, assigned U.S. Ser. No. 08/299,034 and filed Sep. 17, 1993, issued U.S. Pat. No. 5,417,170.
7. "Material Transport Pusher Mechanism In A Material Processing Apparatus" by Roger D. Eshleman, assigned U.S. Ser. No. 08/123,747 and filed Sep. 17, 1993, issued U.S. Pat. No. 5,361,709.
8. "Improved Casing And Heater Configuration In A Material Processing Apparatus" by Roger D. Eshleman, assigned U.S. Ser. No. 08/123,454 and filed Sep. 17, 1993, issued U.S. Pat. No. 5,428,205.
9. "Method of controlling hydrocarbon release rate by maintaining target oxygen concentration in discharge gases" by Paul H. Stevers, assigned U.S. Ser. No. 08/283,118 and filed Jul. 29, 1994, issued U.S. Pat. No. 5,501,159.